1. Field of the Invention
The present invention relates to a thermal assisted type magnetic head.
2. Description of the Related Art
In order to increase recording density of magnetic recording, a recording method has been known. In the recording method, a magnetic anisotropy constant of a magnetic recording medium is increased to enable stable recording on particles with reduced particle size, and simultaneously coercive force of a magnetic recording medium is reduced by focally heating a region on which recording is performed to make recording easier. A magnetic head using such recording method is called a thermal assisted type magnetic head. A region to be heated on a magnetic recording medium need to be smaller than wavelength of light, near-field light is preferably used as a heating method.
U.S. Pat. No. 7,330,404 discloses a technology of matching oscillation frequency of light and resonant frequency of plasmon generated in a metal by irradiating light to a metal scatterer. However, with this method, the metal scatterer that is a near-field light generating element is deformed by excessive heating, so practical use of this method is difficult. As a technology with which such excessive heating can be prevented, in U.S. Pat. No. 7,855,937 and U.S. Pat. No. 8,000,178, a thermal assisted head using surface plasmon polariton coupling has been proposed. The technology described in these specifications uses surface plasmon polariton that is generated on a surface of the near-field light generating element by evanescently coupling light propagating through a waveguide to the near-field light generating element instead of directly irradiating the light to a plasmon antenna. Using surface plasmon enables to suppress that the entire near-field light generating element is heated.
On the other hand, in order to obtain a small magnetization reversal pattern with a high signal-to-noise ratio (S/N ratio), a light spot with high energy and a sufficiently narrowed spot size needs to be formed on a magnetic recording medium. In order to form a light spot with high energy using a structure that light propagating through a waveguide and a near-field light generating element are evanescently coupled, it is needed to generate intense near-field light, and in order to achieve that, it is preferred to enhance coupling efficiency of evanescent coupling of the near-field light generating element and the propagation light. In order to enhance the coupling efficiency, it is preferred that difference between wavenumber of the light propagating through the waveguide and wavenumber of surface plasmon polariton generated in the near-field light generating element becomes not significantly large. However, because spot size of near-field light and the wavenumber are in a negative correlation, the spot size cannot be sufficiently narrowed when mismatch of the wavenumbers is small. When the wavenumber of surface plasmon polariton is increased to narrow the spot size, the coupling efficiency of evanescent coupling is degraded.
The objective of the present invention is to provide a thermal assisted type magnetic head that can generate near-field light with high energy density and a small spot size and that easily suppresses performance degradation due to excessive heating.